Being really good and self publishing helps you build your resume in a superstar industry, but it's nearly impossible to actually achieve commercial success without a well-established sponsor. Adele probably wouldn't have gotten far on her own if XL or some other label hadn't picked her up. Her accomplishment before then was a really good resume.
XL Recordings came from the rave scene, the Prodigy were their first big earning group, however, for every Prodigy there were scores of 'Dome Patrol' grade releases that nobody except for DJs ever heard of. XL moved on when the rave scene died to other acts in different genres. They 'pivoted' to use the parlance.
I know Adele worked with a label however it was a genuine thing her getting spotted on mySpace and not some fake back story, as per the Lily Allen example. I don't believe music executives really would listen if I posted my singing efforts to the internets, or if anyone here did likewise. Yet that did happen with Adele.
I don't think this phenomenon is limited to one form of media. "The Martian" by Andy Weir was originally published on his website one chapter at a time. The internet has, in general, lowered the bar to publishing any kind of work significantly - be it games on Steam, songs on YouTube, or ebooks on Amazon. There are now new avenues into the wider industry, but the big players are still the big players.
Nearest example I can think of, mySpace was where she got noticed so that dates her breakthrough to a good decade ago.
Adele was legitimate however others have pretended to have an Adele grade story. Lily Allen also claimed to have been discovered on mySpace but her dad was rock and roll celebrity so her efforts can be dismissed as nepotism. Often these links to rock and roll celebrity are not obvious because performers use stage names that sound like real names. Their children don't use their parents fake surnames.